The invention relates to medical instrumentation, in particular to diagnostic measuring instrumentation for refractive surgery, and can be used for optometric investigations of vision and high quality laser-based operations of vision correction.
Methods and devices for investigation of aberrations of the optical system of an eye as a function of spatial pupil coordinates are known. Publication of R. H. Webb, et al. Measurement of ocular local wave front distortion with a spatially resolved refractometer. Applied Optics, 1992, Vol. 31, pp. 3678-3686 describes measurement of the optical power of an eye in different points of entrance pupil. The disadvantage of this implementation of Scheiner principle follows from the direct participation of the patient in the procedures of aberration measurements, it means, the measurements are subjective. They require considerable time, tiring the patient, and leading to low accuracy because of unstable accommodation state of the eye, eye movements in the process of measurements, etc.
Methods and devices for objective measurement are known as well. In one of them, described, e.g., in the publication of M. Mrochen, et al. Principles of Tscherning aberrometry. Journal of Refractive Surgery, 2000, Vol. 16, pp. 570-571, a regular light pattern is projected on the retina, its distortions being correlated with aberration parameters of the optical system of the eye. In addition to the disadvantages of technical nature (difficulties with identification of some details of the distorted regular light pattern), the method has a fundamental disadvantage: measured distribution of aberrations is inadequate to the distribution formed by a beam of rays coming from infinity and focused on the retina.
The skiascopic principle is known of projecting moving strips of light on retina. Light backscattered from retina is detected by a set of photodetectors, characteristics of refraction are determined from temporal dependencies in the detected pulse signals for different orientations of the projected light pattern (see for example S. MacRae, et al. Slit skiascopic-guided ablation using the Nidek laser. Journal of Refractive Surgery. 2000, Vol. 16, pp. 576-580). The drawback of this technique is in the difficulty of its realization requiring a large number of movable mechanical parts and still having low resolution of measurements.
According to the ray tracing technique for measurement of refraction aberrations, known from the patent application of Ukraine (V. V. Molebny, et al. Device for measuring aberration refraction of the eye. Patent Application of Ukraine No. 98105286, Int. Cl. A61B 3/00, A61B 3/10, A61B 3/14, filed Oct. 7, 1998, which is now the Ukranian Patent 46,833 published Jun. 17, 2002. See also; U.S. International Patent Application PCT/US99/23327, Int. Cl. A61:3 3/00, filed Oct. 7, 1999, International Publication Number WO 00/19885, Apr. 13, 2002. Ray tracing technique is also a part of the U.S. Pat. No. 6,409,345 to V. Molebny, et al. Issued Jun. 25, 2002.), entrance aperture of the eye is scanned by a narrow laser beam in parallel to the line of patient""s sight, and coordinates of its projection on retina are measured sucessively in time. Map of refraction errors is reconstructed from these data.
For parallel (in time) measurement of wave aberrations, measurement of wave front structure is used at the exit of the eye by means of partitioning this structure into subaperatures. This method, described in the patent issued to D. R. Williams, et al. (Rapid, automatic measurement of the eye""s wave aberration. U.S. Pat. No. 6,199,986. Int. Cl. A61B 3/10, Mar. 13, 2001), is chosen for a prototype. In accordance With said method, a narrow beam of laser radiation is directed into the eye, the component backscattered by the retina is selected from the radiation returning from the eye, this selected radiation is divided into subaperatures by means of a lenslet array, the wave front tilt in each subaperture is determined by measuring the shift of the focal spot position in regards to the optical axis of the corresponding lens of said lenslet array. The wave front is reconstructed from its tilts in separate subapertures and the wave front aberrations are calculated as coefficients at Zernike polynomials, defining the wave front surface.
Difficulties in identification of focal images formed simultaneously in all subaperatures are distinctive for this method. They result in narrower dynamic range of the measured aberrations down to xc2x13 diopters, that is insufficient for practical use. The range could be made wider at the expense of wider subaperatures, but it would result in lower spatial resolution of measurements. In the same manner, making higher the spatial resolution at the expense of larger number of analyzing subapertures would result in narrower dynamic range of aberrations to be measured. To speak shorter, we shall define both these mutually dependent phenomena as the same drawbackxe2x80x94narrow dynamic range
Accordingly, it is an object of the present invention to provide a new method of measurement of wave aberrations of an eye and device for performing the same.
The first invention of the group has an objective of making wider dynamic range of the measured wave aberrations of a human eye, without reducing the number of analyzing subaperatures. This objective is resolved by probing the eye with a narrow beam of laser radiation, selecting the component of the radiation scattered by the retina and exited back from the eye, partitioning said component into subapertures by a lenslet array, measuring the wave front tilt in each subaperture by determining the shift of the position of a focal spot in regards to the optical axis of each lens, reconstructing the wave front in accordance with measured tilts thereof in separate subapertures and calculating aberrations of the wave front as coefficients of Zernike polynomials representing its surface, wherein the wave front tilts in the subapertures are measured several times with a tilt of the beam of laser radiation varied in each subsequent measurement within the angular range between the neighboring subapertures, and the reconstruction of the wave front is performed from the data obtained at all angular positions of the beam, with consideration of the tilts of the beam varied in each measurement.
In each measurement with a varied tilt of the wave front, focal images in the subaperatures are shifted, that is equivalent to another measurement with an additional lenslet array. As a result of several measurements, data are obtained which are equivalent to the data obtainable by means of a lenslet array with a larger number of subaperatures. In this way, while maintaining a wide dynamic range of measured aberrations which corresponds to the selected density of subaperatures, spatial resolution is increased due to the fact that, for the sake of wave front reconstruction, the amount of data on wave front tilts is increased several times, that is equivalent to the several times larger number of subapertures.
Device, implementing the proposed method, considers also the modality of its positioning and orienting as well as controlling the accommodation state of the eye, these procedures are not important from the point of view of the sequence of operations, but are needed for obtaining correct results by means of the device. These components are present also in the prototype (See the above mentioned U.S. Pat. No. 6,199,986, and also in the publication of R. Applegate, et al. Optics of aberroscopy and super vision. Journal of Cataract and Refractive Surgery. 2001, Vol. 27, pp. 1093-1107), containing probing and measuring channels, which are separated by a polarization beam splitter, and a channel of positioning, orientation and providing an accommodation state of the eye. The probing channel is composed of a laser and a telescope functioning as a beam former, while the measuring channel is composed of a relay lens, a lenslet array and a matrix of position-sensing photodetectors installed in their foci, and connected to a signal former, which is linked to a computer performing the functions of the device control, of reconstruction of the wave front and of calculation of wave aberrations.
The disadvantage of the prototype is in its incapability to widen the dynamic range of measured aberrations, for example, at the expense of increase in sizes of analyzing aperatures without reduction of spatial resolution.
To implement the proposed method into a device, an objective is set for said device to increase the dynamic range of measured wave aberrations due to introduction of additional components and links, this objective is to be achieved in two versions of the device.
In the first version of the embodiment, the device for measurement of wave aberrations of an eye, containing a probing channel and a measuring channel, separated from each other by a polarization beam splitter, and a channel of positioning, orientation and providing an accommodation state of the eye, the probing channel being composed of a laser and a telescopic beam former of radiation, the measuring channel being composed of a relay lens, a lenslet array, and a matrix of position-sensing photodetectors installed in their foci and connected to a signal former, which is linked to a computer performing the functions of the device control, of reconstruction of the wave front and of calculation of wave aberrations, wherein the measuring channel, along the path of radiation exiting from the eye, is provided with a unit of dosed tilting of the wave front, said unit of dosed tilting of the wave front being located between the relay lens and the lenslet array, wherein said unit of dosed tilting of the wave front includes a two-coordinate acousto-optic deflector, a telescope-selector, a driver of the acousto-optic deflector and a frequency synthesizer so that the two-coordinate acousto-optic deflector and the telescope-selector are arranged in series, outputs of the driver are connected to the acousto-optic deflector and the output of the frequency synthesizer is connected to the input of the driver, and its controlled input is linked to the computer.
Such a construction of the device, using the unit of dosed tilting of the wave front, which is introduced directly into the measuring channel, allows to vary the tilts in all subaperatures simultaneously and to thus shifting the focal images in each subaperature by the same value that, as a result, is equivalent to the action of several lenslet arrays or, in other words, is equivalent to obtaining data with a lenslet array having a larger number of subapertures.
In the second version of the instrumental embodiment of the proposed method, the control of the wave front tilt of the radiation, exiting from the eye, is performed indirectly due to shifting the laser beam projection on the retina which is a secondary source of radiation. For this purpose, into the device for measurement of wave aberrations of an eye, containing probing and measuring channels, separated from each other by a polarization beam splitter, and a channel of positioning, orientation and providing an accommodation state of the eye, the probing channel being composed of a laser and a telescopic beam former of radiation, the measuring channel being composed of a relay lens, a lenslet array, and a matrix of position-sensing photodetectors installed in their foci and connected to a signal former, which is linked to a computer performing the functions of the device control, of reconstruction of the wave front and of calculation of wave aberrations, namely, into the probing channel, after the telescopic beam former, a unit of dosed tilting of the wave front is introduced, wherein said unit of dosed tilting of the wave front includes a two-coordinate acousto-optic deflector, a telescope-selector, a scan converter, a driver of the acousto-optic deflector and a frequency synthesizer, so that the two-coordinate acousto-optic deflector, the telescope-selector and the scan converter are arranged in series, outputs of the driver are connected to the acousto-optic deflector, the output of the frequency synthesizer is connected to the input of the driver, and its controlled input is linked to the computer.
The device according to the second embodiment is designed in such a way that the probing ray enters the eye always through the same point of the cornea, regardless of the tilt of the ray. The tilted probing ray is projected on the retina with a shift, resulting in the tilt of the exiting radiation. In its other attributes, the second embodiment is the same as the first one, meaning that the tilts of the exiting wave front are performed simultaneously in all subaperatures, due to the unit of dosed tilting of the wave front introduced in the probing channel, so as to provide shifts of the focal images in each subaperature by the same value, which, as a result, is equivalent to the superposition of action of several lenslet arrays, or, in other words, equivalent to obtaining data corresponding to a lenslet array with a larger number of subaperatures.
Due to the proposed construction of the devices for measurement of wave aberrations of the human eye and introduction of the described interactions, the possibility of unambiguous identification is achieved of light spots in the plane of the matrix of position-sensing photodetetors, enabling to avoid essential disadvantage, namely a narrow dynamic range of the measured deviations of the wave front, and to make wider the dynamic range of the measured wave aberrations of the human eye with a given spatial resolution or to increase spatial resolution at a given dynamic range of measured wave aberrations.
The novel features which are considered as characteristic for the present invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.